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Definition of Flexible Printed Circuit Boards
A flexible printed circuit board also called a soft board or a flex PCB, is made of a flexible insulating material circuit board. Its biggest feature is flexibility, able to bend and rotate freely.
Flexible printed circuit board selection of insulation substrate commonly used there are two: polyimide(PI) or polyester(PET) film. Viasion is a trusted flex PCB manufacturer with 16 years of experience for all these flexible circuit material..
- Polyimide is an organic polymer material with excellent comprehensive performance, with high-temperature resistance up to 400°C and high thermal stability and flexibility. Polyimide flex PCB material is the most widely used flex PCB material, such as smartphone, wearable devices, automotive electronics, cameras, automation, robotics, medical, aerospace, military electronics etc. Besides, because of it higher temperature resistance, PI flex PCB material can be used in the dielectric substrate of solar cells and LED lamps and lanterns. The lamps and lanterns will generate a lot of heat under long-term high-temperature conditions, and only the flex circuit can work normally for a long time.
- Polyester film is also a kind of high polymer material, with good mechanical and thermal stability, but cheaper cost than polyimide(PI) material. It is widely used in consumer electronics and automotive industry.
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Types of Flexible Printed Circuit Boards
There are many ways to classify flex PCBs. If we talked about the layer counts, single-sided flexible PCBs, double-sided flexible PCBs, and multi-layer flexible PCBs are common types of flex PCBs.
Flexible circuit boards can also be classified from the flex PCB design perspective, then there are two types of flexible PCBs that can be designed as dynamic or static PCBs.
Here we would like to take more about static and dynamic flexible PCB design. It may be helpful to you which kinds of boards should be used in your devices when doing your flexible circuit designs.
Static design refers to the product encountering the bending or folding only in the assembly process, or the bending or folding which rarely occurs during use. Single-sided and double-sided PCBs like multi-layer printed circuit boards can also be successfully implemented in the folded, static flex PCB design.
Typically, for most double-sided and multi-substrate designs, the minimum bend radius for folding should be ten times the thickness of the entire circuit. Circuits with more layers (eight or more) can become so stiff that it is difficult to bend them. Therefore, for double-sided circuits requiring a tight bend radius, all copper alignments are set on the same side of the substrate film in the folded area. The folded area is approximated to a single-sided circuit by removing the laminate on the opposite side.
Dynamic flex PCB design are used for repeated bending over the entire life cycle of the product. Taking the example of cables for printers and disk drives, as an illustration, to achieve the longest bending life cycle for a dynamic circuit, the relevant part should be designed as a single-sided circuit with copper on the central axis. A central axis is a theoretical plane that is the central layer of the material that makes up the circuit.
By using a substrate film and laminate with the same thickness on both sides of the copper, the copper foil is accurately placed in the center and is subjected to minimal stress during bending or flexing.
The Benefits of Flexible Printed Circuit Boards
- Flexible PCBs are lightweight, so they can be efficiently integrated into electronic components without increasing their size.
- Flexible PCBs are known to dissipate heat efficiently.
- Flexible PCBs are easy to assemble and require less time and cost.
- Flexible PCBs have flexible substrates, making them ideal for assembling tiny electronic parts.
- The use of FPC can greatly reduce the size of electronic products, in line with the development of electronic products to the direction of high density, miniaturization, and high reliability.
Advantages of Flexible Printed Circuit Boards
Flexible PCBs have many advantages compared to rigid PCBs, such as excellent flexibility, light weight, high soldering and usage temperature. Viasion is a flexible PCB manufacturer in China with more than 15 years of experience, please get in touch with our sales team to quote you flexible PCB price.
- Flexible printed circuit board has good flexibility, which means that it can bend and fold freely, and bend millions of times. Flexible PCB above-the-line connection will not be affected.
- Electronic products using flex circuits, in contrast to the rigid printed circuit board, can reduce the volume and weight of electronic products, and adapt to the development trend of electronic products' miniaturization, and portability. Because of the high density of FPCB wiring, the size and volume of FPCB will be smaller when the same wiring is used. At the same time, the flexible printed circuit board will weigh less than the rigid printed circuit board due to the choice of materials.
- The flex circuit has good weldability and heat dissipation performance. Weldability refers to the degree of difficulty of welding. Good weldability means that welding is easier. Poor weldability indicates that welding is difficult and prone to welding problems.
- The flexible printed circuit board has good solderability. Its welding components are not easy to drop or need to be welded. There are fewer fake welding and other quality problems. Once there is no successful welding, this means that the circuit board above the component connection will be a problem, resulting in the flexible printed circuit board can not be used normally. The insulation material used by the flexible printed circuit board is very resistant to high temperatures and can still be used normally at 400 degrees Celsius.
Disadvantages of Flexible Printed Circuit Boards
- The initial cost is high, and the circuit design and wiring cost of the flexible printed circuit board is high. In application, if the number of circuit boards is not very large, try to use rigid printed circuit boards.
- The maintenance of flexible circuit boards is more difficult. There is a protective film on the surface of the flexible printed circuit board.
- The size of the flexible printed circuit board cannot be too long and wide.
- Flexible circuit board is easy to damage. If the factory improperly damages the flex circuit, it needs to pass through the wiring professionals for repair, and the maintenance cost is higher.
Factors Affecting Flexible PCB Welding Quality
The flexible printed circuit board is composed of the plate and the above components. The components of the welding process are critical. Any improper operation, or poor connection, is likely to lead to the malfunction of the components of the flexible circuit board, even leading to factory production of a whole bunch of PCB scrap, which is a big loss to the company and customers. So in the production process, we must pay attention to welding this key step. The following is about the impact of welding on several factors:
Reasons and Improvement of Flexible PCB Deformation
As mentioned before, the deformation of the flex printed circuit board will seriously affect the quality of component welding, and the normal use of the flex printed circuit board. What are the factors leading to the deformation of the flex PCB?
- The copper surface area on the flex printed circuit board is uneven The common flex PCB above is covered by green copper foil. If the copper foil distribution of a single flex printed circuit board is not uniform, it will result in inconsistent heat absorption and dissipation of the board, because thermal expansion and cold contraction are the nature of all solids. The copper foil is not corresponding, which indirectly leads to inconsistent thermal shrinkage. At this time, if the flex printed circuit board just exceeds the TG value, it will cause permanent deformation of the flex printed circuit board.
- Connection points of each layer on the flex printed circuit board (vias, holes) There are many layers of the flex PCB, and the connection point between layers will affect the thermal expansion and cold contraction and cause deformation of the flex printed circuit boards.
- Weight of the flex printed circuit board itself. If the size of the flex PCB is too large, or the components on it are too heavy, the flex printed circuit board will cause middle depression because of the heavy weight in the welding furnace.
- V-Cut - Some customers will require the addition of a V-cut on the flex printed circuit board. This method is not worth the loss, because V-cut is easy to cause deformation.
Given the above causes of circuit board deformation, what measures should we take?
- Reduce the impact of temperature on board stress. Thermal expansion and cold contraction will cause deformation of the plate, so we try to keep the temperature of the welding furnace under constant conditions, and if possible, try to keep the temperature lower to prevent it from exceeding the TG value.
- Use a high TG plate. TG value is thermal conductivity. The low TG value of the circuit board is easy to deform due to high temperature. Manufacturers try to use a high TG value for circuit board production, but it is noted that the general TG value is higher, and the price is relatively high. It’s better to confirm with customers about that in advance.
- Increase the thickness of the flex PCB. Now electronic products are toward miniaturization and frivolous development. Some flex printed circuit board manufacturers try to meet the needs of consumers, and carry out the production of the ultra-thin circuit board. The thickness of some flex printed circuit boards is less than 1 mm. It is difficult to prevent deformation for thin circuit boards in welding furnace heating. It is suggested that customers choose a thicker flex printed circuit board.
- Reduce the size of flex PCB. If the circuit board size is too large, the corresponding weight is bigger, which will cause deformation in the circuit board in the middle, probably because of the gravity sag. To avoid the happening of this kind of situation, the circuit board size should be smaller as far as possible.
- Use furnace tray fixture. If the scale of the printed circuit or the assembly can't be modified, boiler trays are often used. If the on-top of strategies square measure is tough to attain, the last item to try is to use a reflow carrier/template to cut back the deformation. The reflow carrier/ template will cut back the bending and warp of the plate. The rationale is that whether it is hot expansion or cold contraction, it's hoped that the receptacle will fix the printed circuit till the temperature of the printed circuit is below the TG worth and starts to harden once more.
- If the single-layer receptacle can't cut back the deformation of the printed circuit, it's necessary to feature a layer of the canopy, the printed circuit with the higher and lower 2 layers of receptacle clamp, so the printed circuit will greatly cut back the deformation drawback of the attachment chamber. However, this chamber receptacle is sort of expensive and even has to feature manual placement and utilization receptacle.
- Don't use V-cut. As V-cut might cause deformation of the flex printed circuit board, it's best to not use V-cut within the production of the flex PCB. If the client needs V-cut, the V-cut ought to be as shallow as the potential. Viasion is a trusted flex PCB manufacturer since year 2007, please contact us any time for a free quote of your flex PCB cost.
Circuit Board V-cut and Its Residual Thickness and Angle Requirements
V-cut is the dividing line cut by a knife on the edge of the flex printed circuit board by the circuit board company for future assembling. Its shape is similar to the letter V, so it is called V-cut. V-cut is rarely used in flex PCBs, but possible to use in rigid-flex PCBs.
V-cut thickness refers to the remaining thickness of the flex printed circuit board after it is Cut. This index is very critical and determines whether the flex PCB is easy to be broken or deformed.
Conventional requirements for Chinese circuit boards are that the residual thickness is one-third of the total thickness of circuit boards, but greater than 0.35 mm, not greater than 0.8 mm, meet these three parameters, the residual thickness is too small is not good, because if the residual thickness is too small, easy to break; The residual thickness is too large, too strong, and then it’s harder to break.
The Angle of V-cut is the Angle of the slot Cut on the rigid-flex PCBs, and there are three kinds: 30 degrees, 45 degrees, and 60 degrees. The most common is 45 degrees.
The Angle size has advantages and disadvantages, if the Angle is larger, it means that the rigid flex printed circuit board edge is cut off the circuit board by the cutting knife, for normal connection of the line, the line must move a little, so as not to be cut off the line by the cutting knife. If the Angle is smaller, on the one hand, the line does not have to move inside, on the other hand, this requires that the blade of the cutting knife is thinner, wears faster, and even the blade is broken. Therefore, the Angle of the V-cut should be moderate. Not too big, not too small.
Thermal Conductivity of the Flex Printed Circuit Boards
When the temperature of the card rises to an exact extent, the solid-state is reborn into a rubber state, and therefore the temperature at this point is termed the thermal physical phenomenon of the card. brought up as TG, it is far-famed that the thermal physical phenomenon is the highest temperature within the rigid state of the card. higher than this temperature, the card becomes soft and its mechanical and different properties decline apace.
There square measure 3 levels of TG, the final grade is a hundred thirty degrees, the medium grade is a hundred and fifty degrees, and therefore the advanced grade is a hundred and seventy degrees. The thermal physical phenomenon of the card is improved, which suggests that prime temperature resistance, stability, and different properties square measure higher.
With the rapid development of electronic products, especially consumer goods such as mobile phones and computers, the volume of data transmission is increasing and the transmission speed is getting faster and faster, which leads to the long-term high-load operation of flex printed circuit boards and the generation of a large amount of heat. Objectively, circuit boards are required to have good heat dissipation and high-temperature resistance, which leads to the increasing demand for high thermal conductivity circuit boards.
According to the experimental data, the general circuit board and high thermal conductivity circuit board, in a high-temperature environment, the mechanical performance of the circuit board, stability, heat dissipation, and other indicators are very different, the high thermal conductivity circuit board is better than the general circuit board.
In recent years, the quantity of shoppers buying high Tg boards has magnified year by year.
Application of Flexible Printed Circuit Boards
There are three main application fields of flexible printed circuit boards in China: automotive electronics, wearable devices, and smartphones etc.
- The size of the smartphone is not large, but there are 10-15 pieces of the flexible circuit board in a smartphone. The flexible printed circuit board is used in many modules of mobile phones, such as the battery of the mobile phone, display, connection module, touch module, and camera. Many phones have four cameras on the back. It is believed that with the promotion of mobile phones to miniaturization and folding mobile phones, smartphones will need more and more flexible printed circuit boards.
- In the field of wearable devices, it is not convenient for people to carry mobile phones when they are doing outdoor sports such as running, so many people choose smart watches, smart bracelets, and other wearable devices. On the one hand, it can have part of the functions of a mobile phone to maintain communication with the outside world. On the other hand, it can record running data, such as heart rate, distance, and time of running. The popularity of wearable devices will greatly boost the sales of flexible printed circuit boards.
- Automotive electronics: With the advent of the era of electric vehicles and driverless driving, the electronic and intelligent degree of automobiles is becoming higher and higher.
- The demand for flexible circuit board in automotive sensors and LCD screens is also increasing, which brings a broad space for the development of flexible printed circuit boards.
Manufacturing Capabilities of Flexible Printed Circuit Boards in Viasion
Viasion is one of the most trusted flex PCB manufacturers in China, please refer to our manufacturing capabilities as following:
- 1 -8 Layer flex PCBs, 2-20 layer rigid flex PCBs
- Material: polyimide, PET, FR4 ( including Semi-flex)
- Stiffener: unclad FR4, Polyimide, metal, PSA
- Silver film & sliver ink EMI shielding
- Custom Surface Finishes Available
- Flex PCBs type: single-sided flex PCB; double-sided flex PCB, multi layer flexible PCBs, heavy copper flex circuit boards
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Frequently Asked Questions
Flex circuit has the advantages of high wiring density, lightweight, thin thickness, bendability, flexibility, etc.
Firstly, it may withstand millions of dynamic bending without damaging the wire, move and expand at will according to the requirements of space layout, realize three-dimensional assembly, and achieve the effect of integration of component assembly and wire connection.
Next, flexible PCBs have excellent electrical properties. The lower dielectric constant allows for fast electrical signal transmission; the good thermal properties make it easy to cool down the components; the higher glass transition temperature or melting point allows the components to operate well at higher temperatures.
Furthermore, since we can assemble most of the components of rigid parts, and the flex part is used for connection, the assembly reliability and yield of flexible PCBs are higher.
Manufacturing flexible PCBs is challenging for the following reasons.
1) The costs for the design and wiring of flexible PCBs are high.
2) The production process of flexible PCBs is complex.
3) The maintenance of flexible PCB is difficult and the maintenance cost is expensive.
4) The base material used in flexible PCBs are more expensive than FR4 material;
5) Drill hole treatment and copper plating in flexible PCBs fabrication is more difficult.
Yes, we supply single layer flex PCB, double layer flex boards and multilayer flex printed circuits up to 10 layers. But normally flex boards only have 1-4 layers, since flex circuits with higher layer boards are not flexible enough. So high layer count flex boards are designed as rigid flex PCBs.
Flexible PCBs are widely used in mechanically sensitive equipment because of their flexibility to resist vibration. Their application areas are described below.
1) Computers and accessories
2) Aircraft, drones, cameras
3) Automobiles
4) Cell phones
5) Medical electronics
6) Wearable devices
8) Industrial applications: For example, flexible PCBs are also used in industrial machine and power distribution control systems
8) Military equipment
9) Aerospace
With the continuous promotion of information technology and intelligent construction, the market demand for flexible printed circuit boards will also become greater and greater. It is already difficult to find slightly complex electronic products that do not use flexible PCBs as well as flexible PCB assembly.
As China will further increase its investment in various fields of electronic information construction, the pace of electronic information construction in downstream areas is bound to drive the development of the flexible PCB & PCB assembly industry. In addition, with the emergence of new electronic products, the market and applications of flexible printed circuit boards will also be expanded, and the renewal of different categories of electronic products will bring more considerable development prospects than the traditional market. Particularly, flexible PCBs have a promising market in satellite and avionics devices, high-end sensors, flexible heat-generating components, medical devices, remote control devices, and security devices.
Flex PCBs is produced with only flexible PCB materials, such as PI and PET, while rigid flexible PCB combine both rigid material (normally FR4 material) and flexible material together. So on flex boards, the whole flexible circuit boards can be bent; but on a rigid flex PCB, only a certain part where these is flexible material can be bent.
As we know that every flexible printed circuit board should have a custom flex PCB design, here is a typical stack up for a normal 2 layer flex PCB:
–cover layer 25um
–adhesive: 25 um
–top copper layer: 35 um
–Polyimide (PI) base material: 25 um (or 50 um)
–bottom copper layer: 35 um
–adhesive: 25 um
–cover layer 25 um